This invention relates to a microcomputer with a built in character display circuit (hereinafter called the OSD (onscreen display) circuit) for displaying character information on a display screen, and to a visual display unit that employs such a microcomputer.
In order to achieve improvements in device operability and to display various information, AV (audio/video) devices, such as television receivers and video cassette recorders, contain a microcomputer with an OSD capability to display character information on a display screen (e.g., a CRT display and an LCD (liquid crystal display)).
A typical conventional microcomputer with a built in OSD circuit is described. Generally, such a conventional microcomputer comprises a CPU (central processing unit) for executing instructions, a ROM (read only memory) for storing CPU instructions, a RAM (random access memory) which stores CPU data or serves as a stack region, an OSD circuit for displaying character information on a display screen, a ROM for storing font data of characters, and a RAM for storing character information including (i) character codes representing the characters and (ii) attribute data of the characters such as character size data and character color data. In such a conventional microcomputer architecture, the CPU requires its own ROM and RAM while the OSD circuit also requires its own ROM and RAM. This results in an increase in chip area. Another problem is that since ROMs and RAMs necessary for CPUs are different in storage capacity from ROMs and RAMs necessary for OSD circuits with respect to the grade of AV devices, to the destination thereof, and to required OSD capabilities, numerous types of products are required to be prepared when trying to achieve an optimal design according to various specifications.